1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point with a wire.
2. Prior Art
Various wire bonding methods have been proposed in producing semiconductor devices. The most general method is illustrated in FIG. 2.
First, in step (a), a ball 2a is created by electrical discharge of an electric torch 3 on a wire 2 that extends from the lower end of a capillary 1. Afterward, the electric torch 3 is moved in the direction shown by the arrow. As shown in step (b), the capillary 1 is then moved to a point above the first bonding point 4a. In step (c), the capillary 1 is lowered, and the ball 2a at the tip end of the wire 2 is subjected to a first bonding to the first bonding point 4a.
Afterward, in step (d), the capillary 1 is raised. Then, in step (e), the capillary 1 is moved to a point above the second bonding point 5a, which is located on a lead 5. Next, in step (f), the capillary 1 is lowered, and the wire 2 is subjected to a second bonding to the second bonding point 5a. After the capillary 1 is raised to a predetermined position, a damper 6 is closed; and the capillary 1 and damper 6 are raised together so that the wire 2 is tail-cut from the attachment root of the second bonding point 5a in step (g).
One wire connection is thus completed.
Examples of this wire bonding method are disclosed in Japanese Patent Application Laid-Open (Kokai) No. S57-87143 and Japanese Patent Application Publication (Kokoku) No. H1-26531.
In Japanese Patent Application Laid-Open (Kokai) No. S57-87143, after finishing bonding to a first bonding point, a capillary is raised for a distance that is equal to the length required for bonding to a second bonding point, and then the capillary is moved along a circular path to the second bonding point or a point adjacent to the second boning point.
In Japanese Patent Application Publication (Kokoku) No. H1-26531, after the completion of bonding to a first bonding point, a capillary is moved in the opposite direction from the second bonding point, and then the capillary is moved along an oblique upward path and then oblique downward path so as to be brought to a second bonding point.
FIGS. 3(a)and 3(b)show the end of capillary in step (f) of FIG. 2 and the end of the wire in step (g) of FIG. 2.
As seen from step (f) and FIG. 3(a), the wire 2 is pressed against the second bonding point 5a on the lead 5; and in step (g), the wire 2 is tail-cut from the attachment root thereof. As a result, a projection 2c which is bent laterally remains at the tip end of the tail 2b (i.e., the portion of the wire 2 that extends out to the capillary 1) as shown in FIG. 3(b). As seen from FIG. 3(a), the projection 2c is formed when the wire 2 is pressed against the lead 5; and when the wire is thus pressed against the lead 5, impurities 7 on the surface of the lead 5 adhere to the undersurface of the projection 2c.
If the ball 2a is formed at the end of the wire with impurities 7 adhering thereto, the impurities 7 remain on the ball 2a, resulting in that the discharge efficiency changes, and abnormal balls are formed at the end of the wire. Furthermore, it is difficult to form a small ball. When the volume of the ball 2a formed is large enough to absorb the volume of the projection 2c, eccentricity would not occur in the ball 2a even if the projection 2c extends laterally at the end of the wire. However, when the ball 2a formed is small and is of the size close to the volume of the projection 2c, then the formed ball 2a is eccentric as shown in FIG. 3(c). In this regard as well, the formation of a small ball is difficult.
In recent years, so as to accomplish a reduction in the cost of dice (semiconductor elements), there has been a tendency to reduce the size of dice and to use a fine pitch bonding so that the spacing of pads is narrow and as small as approximately 50 to 60 microns. In conventional wire bonding methods, however, it is difficult to accomplish a formation of extremely small balls that are free of eccentricity as described above on the consistent basis. As a result, the conventional methods are not sufficiently able to comply with fine pitch bonding.